The preferred embodiment relates to a method and device for function-monitoring an actuator in a process engineering plant, particularly during operation of the plant, the actuator comprising a valve, an actuating drive, such as a pneumatic actuating drive, for actuating the valve and a positioner for setting the position of the valve.
Particularly the operation of a process engineering plant involving toxic fluids poses a risk to man and his environment that needs to be monitored. For this purpose, use is made of actuators including a safety valve which in an emergency situation automatically assumes a safe position. To comply with stringent safety standards, plant operators always need to be sure of correct functioning of the actuators. This is usually done by a test run of the safety valve to a safe position which with most actuators necessitates shutting down at least part of the plant. Thus on-line monitoring, i.e. during its operation, would be desirable. Function-monitoring for this purpose should take into account as many components of the actuator as possible which are susceptible to failure.
One method of function-monitoring an actuator having a proven record of success is described in the German article entitled “Neues integriertes Diagnosekonzept bei digitalen Stellungsreglern” by Dr. Jörg Kiesbauer as published in the magazine “atp-Automatisierungstechnische Praxis”, year 46, 2004, No. 4/2004 involving a novel integrated diagnostic concept for function-monitoring digital positioners. In this arrangement, functioning of the actuator is monitored mainly by making use of a displacement sensor which senses an actual value for comparison with a set point value of the position. When the discrepancy between actual and set point position is prohibitive, malfunctioning of the actuator is signalled.
It is known from experience that the internal friction such as sliding friction particularly with closed-loop control valves and sticking with safety control valves in the actuating drive and in the valve is a parameter providing a strong indication as to functioning performance of the actuator. How good the closed-loop control is depends on the existing friction, i.e. the lower and more uniform the friction, the higher the accuracy in attaining closed-loop control.
With safety valves or shutoff valves the forces needed to position the safety valve into its safe position are usually produced by a preloaded spring or by a source of pneumatic energy. If the safety valve has not been operated for a long time, the valve element is subject to high sticking forces, with the risk of the valve element failing to attain the safe position in an emergency situation, because the sticking forces exceed the driving forces as made available or because the actuator upon overcoming the sticking is caused to over-shoot. The problems involved in known methods of function-monitoring may result in safe operation of the plant being seriously diminished.
Even with closed-loop control valves, which are often positioned, the problem of sticking or uneven friction may occur, making it essential to sense the true displacement of the closed-loop control valve. Once the monitoring procedure has been started, sticking or slipping of the valve element cannot be sensed until the resistance has been overcome and the high drive forces needed for this purpose act on the valve element which inevitably results in an undesirable overshoot of the valve, termed stick/slip.